The present invention relates to a semiconductor memory device, and particularly to a dynamic type semiconductor memory device employing field effect transistors.
Memory capacity of dynamic type semiconductor memories has been increasing remarkably and 256K-bit dynamic memories have become commercially available. Accompanied by such increase in memory capacity, problem of array noise has been becoming serious. As, is well known in the art, dynamic type semiconductor memories employ so-called one-transistor memory cells, each of which is composed of a capacitor and a transfer gate transistor coupled betwen the capacitor and a bit line. Reading-out of information stored in a memory cell is performed by sensing a small amplitude potential which is produced by capacitive-division of a capacitance of the cell capacitor and the bit line capacitance.
The array-noise is usually caused by capacitive coupling among bit lines in memory cell array, and operations of sense amplifiers affect from each other through the capacitive coupling. For example, most of sense amplifiers amplify one of binary logic levels and the remaining small number of sense amplifiers amplify the other of the binary logic levels and the remaining small number of sense amplifiers amplify the other of the binary logic levels, the remaining small number of sense amplifiers are inevitably affected towards one of the binary logic levels by the most of sense amplifiers.
According to the recent tendency of large memory capacity, the bit line capacitance is increased because of increase in number of memory cells coupled to each bit line while the capacitance of each memory cell capacitors has inevitably become small. Accordingly, the small amplitude signal read-out on the bit line from the addressed memory cell has been further becoming small, and on the other hand amount of the array-noise has increased by the increase in memory capacity. As a result, the substantial signal generated on the bit line from the addressed memory cell has become small in view of the array-noise, resulting in difficult detection of stored information.